The present invention relates generally to the field of ecological management and more particularly to the field of dust control or abatement of saline soils dominated by sodium salts using minimal water resources. One embodiment of the invention was developed for the bed of Owens Lake in Eastern California (FIG. 1) that is dominated by carbonate and sulfate salts of sodium; however, the embodiment described herein is applicable to any environment that presents salt chemistry similar to that within the Owens Lake bed, including for example, other saline dry lakes or sumps receiving and concentrating water from agricultural drainage.
Soils enriched with sodium carbonate and sodium sulfate through agricultural drainage, similar salty beds exposed due to anthropomorphically-changed hydrology (like the Owens Lake), and natural dry lakebeds with shallow groundwater connection are prone to create large sources of airborne dust that can cause health and safety hazards within the surrounding region. Owens Lake was the historic terminus of the Owens River that was diverted for export by the City of Los Angeles resulting in desiccation of the lake early in the last century.
The Owens Lake bed is highly saline with some locations containing up to around 60% by weight of the salts sodium carbonate, sodium bicarbonate and sodium sulfate. When these salts are dissolved by rain or snow during cold temperatures, they re-precipitate as decahydrate that incorporates 10 molecules of water for each salt molecule. The temperatures governing re-precipitation that incorporates decahydrate occurs at about 50 degrees Fahrenheit for sodium carbonate salts and about 65 degrees Fahrenheit for sodium sulfate salts.
Re-precipitation of salts in the decahydrate form causes crystals to swell 4-5 times their volume. Decahydrate salt crystals lose water molecules in alternating warm and cold temperatures in winter and especially during warm sunny days. This process destroys soil cohesion and renders the surface easily lofted by only moderate winds of about 15 miles per hour. This salt-phase-change mechanism is largely responsible for the severe dust problems at Owens Lake, prompting it to be the recognized as the largest single dust source in the United States.
Federal and state laws mandate that the City of Los Angeles Department of Water and Power (LADWP) perform dust control for Owens Lake recognized as the former single largest source of respirable particulate air pollution in the United States. Through several decades of intensive study, three dust control methods have been identified by the agency responsible for monitoring and enforcing dust control: wetting the surface, covering the surface with vegetation, or covering the surface with gravel. Of these three, only surface wetting, in constructed artificial (man-made) wetting basins has been able to accomplish dust control within the time and scale required.
Unfortunately, wetting of the Owens Lake surface is using enormous amounts of water—LADWP uses four feet per year to plan for the required water application for dust control through the dust control season. The total amount of water use can greatly exceed 100,000 acre feet per year on over 40 square miles of the lakebed—this amount is sufficient water to supply about 400,000 families. Within the critically water-short semi-arid southeastern California region, this annual consumption of water for dust control is not sustainable.
During the process of desiccation, naturally saline Owens Lake water concentrates to form an evaporite deposit in the lakebed's lowest topography. The evaporite deposit covers about 34 square miles, has an average depth of about 2.6 feet and varies in thickness from a few inches to about 9 feet (FIG. 2). The evaporite deposit consists of precipitated salts and salt held in concentrated aqueous solution. This aqueous solution is brine, dominated by sodium chloride that remains in solution because it is much more soluble than sodium sulfate and sodium carbonate at common ambient temperatures at the Owens Lake. The sodium chloride-rich brine isolates the potentially emissive salts of sodium carbonate and sodium sulfate from atmospheric desiccation, thereby protecting the source deposit from being dust emissive. The term “wetting basin” is synonymous with “dust control wetting basin” and is a polygonal area enclosed by berms flooded with water to control dust emissions from the lakebed as seen in FIG. 2.
The present invention includes systems and/or methods that will protect the surface from releasing dust, use present infrastructure, if available, that was built to provide surface wetting (or at other sites, create such infrastructure) with minor modification, consume minimal water resources for startup, and consume little or no additional water for maintenance. Each of three methods provides the surface protection by working with the natural properties of the salts present within the Owens Lake system. In one of the methods a different type of salt is imported that, with another ingredient, works to stabilize the surface.
The embodiments of the present invention comprise three methods:                (1) The use of brine from salt mined, dissolved and moved from a natural source deposit to create salt deposits within reconfigured wetting basins, including basins that were originally created for dust control using fresh water flooding. These created salt deposits will form a “Brine Membrane” that mimics the stable non-dust-emissive source deposit by having horizontal beds of precipitated salts with the most soluble salts retained within a sodium chloride brine solution that bathes and caps the precipitated horizontal beds of salt beneath. The control of evaporation by the Brine Membrane maintains the deposit in a wetted state (non-desiccated, non-dust emissive).        (2) The use of small quantities of polymer and divalent cation salts of chloride (for example calcium chloride) to stabilize emissive areas of exposed lakebed, not yet treated by construction and flooding of wetting basins. This is called the “Interim Solution” since it allows for later conversion to a wetting basin or to conversion by the Brine Membrane method once the wetting basin is constructed. The Interim Solution can provide stability on very large areas and can provide protection for wetting basins that were formerly filled with fresh water for dust control.        (3) The use of measurements of soil temperatures to predict when the soil temperature within flooded wetting basins reaches and exceeds the governing temperature when the salts of sodium sulfate and carbonate will no longer undergo phase changes that would render a desiccating surface prone to windborne dust emission. This method is called “Springtime Conservation” because it curtails water to control the dust in wetting basins as the temperatures warm during the spring.All three of these methods provide a transformation from the current wasteful practice of flooding emissive surfaces with large amounts of water that evaporates annually, while protecting the lakebed surface from windborne dust emission.        
A brief description of the three methods follows.